1. The present invention relates to a laser diode driver (hereafter denoted as LD driver) and an optical transmitter providing the LD driver.
2. Related Prior Art
One type of techniques to driver the LD has been well known as the shunt driver, in which an active device such as a transistor is connected in parallel to the LD. U.S. Pat. No. 6,618,408, has disclosed such an LD driver where the LD is modulated by driving a Field Effect Transistor (FET) connected in parallel to the LD. A Japanese Patent Application published as JP-2008-034479A has also disclosed such a shunt driver circuit. Because the active device such as an FET has a substantial gain, the signal to modulate the active device may be reduced in the shunt drive configuration, which may consequently reduce the power consumption.
It has been also known in the field that the shunt drive configuration degrade the optical waveform output from the LD compared to a case where the LD is driven by other configuration such as a series driver. Anther U.S. Pat. No. 6,373,346, has disclosed a technique to improve the output waveform of the LD by a driver able to pre-emphasize or de-emphasize the driving signal, in which the driver generates a positive or negative peaking signal from the input signal and superposes this peaking signal on the input signal. The optical waveform output from the LD may be adjusted by using the pre-emphasized or de-emphasized modulation signal even when the LD is shunt-driven.
In another aspect, the optical waveform output from the LD intrinsically shows a faster leading edge and a relatively slower falling edge due to the carrier relaxation mechanism within the active region of the LD. In another words, the optical output of the LD shows an inductive leading edge and a capacitive falling edge. Because the pre-emphasizing or de-emphasizing of the driving signal only provides a driving signal with a leading edge symmetrical to a falling edge, the asymmetry of the optical waveform of the LD may not be compensated. Still another United States Patent, U.S. Pat. No. 7,147,387, has disclosed an LD driver that enables to generate a driving signal with an asymmetrical leading and falling edges. The technique disclosed therein; (1) makes two intermediate signals, one of which is a difference between an input signal and another signal delayed by the input signal, another one is an exclusive-OR of the two signals mentioned above; and (2) combines thus formed two signals with a preset ratio.
When the shunt driver circuit implemented with an FET is applied to drive the vertical cavity surface emitting laser diode (VCSEL), the response appeared in the driving current for the LD degrades due to input capacitance of the FET, because the shut driver linearly amplifies the voltage driving signal provided therein. A resonant characteristic inherently appeared in an optical rising edge of the LD may compensate the degradation above; while, the degradation in the optical falling edge is enhanced to a substantial level when the transmission speed becomes 10 Gbps or higher.